Missing a lock(mPipelineCache).

Bug: b/168744561
Change-Id: I7e57f9feed7431d0fdfec153e7e8e47fe9a2fa0e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2424463Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>